This invention relates to apparatus for indicating the farthest advance position of a bi-directionally movable medium and, more particularly, to solid-state electronic farthest advance apparatus which can be used, for example, in conjunction with a reversible web without requiring a direct sensing of the condition of the web.
In many processing systems, such as a web processing system, it often is necessary to bi-directionally move the web relative to a processing station. If the web is moved normally in a forward direction, it is desirable, once the web is reversed, to indicate when the web has been returned to its previous farthest advance position.
One type of web processing system wherein knowledge of the farthest advance position of the web is desirable is a dictating system. As one example, in a central dictation system, a central recording unit is adapted to be connected to individual ones of plural remote dictate stations. Once this connection is obtained, an operator at the remote station can dictate messages onto the recording medium at the central recording station and can control various functions, such as dictate, rewind, playback, and the like. In many applications using this central system, a subsequent operator at another remote station can dictate messages onto the recording medium following the completion of the dictation operation by the previous operator. It has been found that, in some instances, the last action taken by the previous operator prior to disconnecting his remote station from the central recording station is to partially review his dictated message. Hence, at the time of disconnection, the record medium has not been fully restored to the farthest advance position corresponding to the end of the dictated message. Therefore, to prevent the subsequent operator from dictating his message over the remaining portion of the previously dictated message, it is necessary to return the medium to its farthest advance position prior to effecting a connection between the remote station of the subsequent operator and the central recording station.
In a central dictation system of the type disclosed in U.S. Pat. No. 3,823,274, assigned to the assignee of the instant invention, the recording medium is an endless loop of magnetic tape capable of being stored in serpentine folds in a housing. A record/playback transducer is provided in the housing and the tape is driven bi-directionally past the transducer. During forward movement of the tape, as in a recording mode, the tape is drawn taut over various guide rollers. However, when the tape is reversed, this tautness is relaxed and a loop is formed between a set of guide rollers. Consequently, to determine whether the tape has reached its farthest advance position, it merely is necessary to sense the tautness of the tape. Thus, when all of the rewound tape has been transported in the forward direction, as during the playback of a previously recorded message, the tape once again is taut. As disclosed in U.S. Pat. No. 3,823,274, the tautness of the tape is sensed by a mechanical switch that is closed when the tape is rewound to form the loop and that is opened when the tape is taut, thereby indicating that the tape has been restored to its farthest advance position.
Although this technique of using a mechanical switch to detect the tape tautness as an indication of the farthest advance position is satisfactory for a central system provided with an endless loop of tape, it is not completely successful when used with other recording media. For example, if the recording medium is a magnetic tape housed in a cassette, reversal of the tape from its farthest advance position does not result in the formation of a tape loop as described above. Although the amount of tape payed out from the cassette supply reel onto the take-up reel can be sensed by measuring the diameter of the take-up reel and then sensing when this diameter is reduced, as during rewind, and subsequently restored to indicate that the tape has returned to its farthest advance position, such reel-measuring is highly inaccurate.
As another example, if the recording medium is a belt that is scanned by a traversing record/playback head, a review operation is performed by moving the head backward relative to the belt. Hence, farthest advance cannot be detected by sensing the tautness or slackness of the medium as disclosed in the aforenoted patent.